Double dose of vaccine protects from chicken pox, study shows

Yale researchers have found that two needles are better than one — at least when vaccinating against chicken pox.

Led by School of Medicine pediatrics and epidemiology professor Eugene D. Shapiro ’70, a study by Yale and Columbia researchers published in the Feb. 1 issue of the Journal of Infectious Disease found that children with two doses of the varicella vaccine are 95 percent less likely to develop chicken pox than those with only a single dose. Shapiro said the study confirmed the soundness of the Centers for Disease Control and Prevention’s move, in 2006, to increase its recommendation of one dose of the vaccine, which had been in place since 1995, to two doses.

Yale Daily News

Yale researchers have verified a two-dose policy that was instated in 2006.

“We were not surprised by the results,” Shapiro said.

Although studies in the past have shown that receiving two doses of the varicella vaccine was associated with greater resistance to the disease in controlled conditions, no data from the general population had been collected on the effectiveness of taking two doses of the vaccine prior to Shapiro’s study.

The study involved 71 case subjects — children four years of age or older enrolled at one of the 28 pediatric practices involved with the study — and found that two doses of the varicella vaccine was 98.3 percent effective at preventing chicken pox. The CDC currently recommends that children aged 12 months to 12 years of age receive two doses of chickenpox vaccine administered at least three months apart.

But some professors are not completely convinced by the study’s results. David Kimberlin, a professor of pediatrics at the University of Alabama at Birmingham, wrote in the same issue of the Journal of Infectious Disease that the study did not distinguish whether two doses of the vaccine are more effective because the first dose is ineffective to begin with or because the second dose provides a boost to the waning effects of the initial dose.

“Given the high effectiveness demonstrated in this trial, however, this distinction is more of an academic exercise than a clinical conundrum,” Kimberlin said. “What matters is that two doses work.”

When asked about the possibility of administering three doses of the vaccine, Shapiro said while it would likely be even more effective than two, it would not be worth the cost given the already high efficacy of two dosages.

Jane Seward, acting director of the division of viral diseases at the National Center for Immunization and Respiratory Diseases, which advises the CDC, concurred with Shapiro’s analysis, praising the “strong methodological study design.”

Still, given the CDC’s recommendation has only been in effect for four years, Shapiro said it would take further monitoring to ensure that the efficacy of two dosages is sustained in the long-term.

“We have applied to get support to continue the research and were not successful,” he said. “But that is certainly an area we want to look into.”

Seward said that based on state health department reports of varicella cases sent to her organization, there had been fewer outbreaks of chicken pox around the nation since the 2006 change in CDC recommendation. She added that monitoring would be ongoing, and that studies like Shapiro’s were very important to corroborate the data collected from the surveillance system already in place.